In drilling a hole, it is essential to know with precision the position of the rotating or percussive tool as well as the instantaneous speed of advance of drilling for a given section of ground. In fact, such information gives multiple data both on the direction of the drilling operations and on the geology of the ground traversed (lithology, tectonics, mineral associations, approach of a metal or hydrocarbon deposit) by the interpretation of the rheological properties of the formations encountered.
The present invention, in addition to geological surveying and monitoring, permits continuing and precise monitoring of the nature of the ground traversed by the drill hole.
The various prior art devices used to date are generally constituted by a mechanical or optical meter, fast with a pulley driven without slide by a cable connected to the upper end of the boring rods bearing the drilling tool.
Certain of these devices record the time taken to drill a certain unit of length, either as a function of time as in U.S. Pat. No. 2,883,256, or as a function of depth as in U.S. Pat. No. 2,981,102.
In these devices, the speed of advance is determined from incremental measurements which correspond to a length frequency of which the transmission, via a system of the frequency-voltage converter type, furnishes a value of the speed of advance. This process involves smoothing out of the analog output signal, affecting the calculation of the measurement by a detrimental error upon considerable variations in speed due to the pass band imposed by the smoothing.